We characterize the operating modes of a magnetized cold cathode plasma in helium at pressures 50-6400 mTorr (6.7-850 Pa). We find at least three distinct modes: (i) a diffuse magnetron plasma with representative plasma voltage 250 V; (ii) a constricted mode with a bright spot at the cathode surface that moves slowly, with representative plasma voltage 50 V; and, (iii) a constricted mode (CO) with a bright spot at the cathode surface that orbits rapidly around the racetrack region defined by the magnetic field, with representative plasma voltage 80 V. This latter mode is of interest because it appears to be a cold-cathode plasma operating at unusually low voltage, and it is associated with a low rate of cathode massloss. We find that the CO mode appears only if the initial rate of current-rise is high, and if the cathode surface is smooth and featureless. It also appears to be correlated with cathode materials that form strong oxide films that can dramatically increase the field emission of electrons from the cathode surface.

Magneticallyenhanced discharge plasmas in light monatomic gases such as helium may have applicability in future electric power grid switches. For practical use of a gas discharge switch in power conversion the net cathode erosion rate must be sufficiently low to enable stable operation over an extended life. We have found that magneticallyenhanced He plasmas at pressures of 200-800 mTorr can operate in one of several distinct modes that are characterized by the plasma geometry (diffuse, constricted) and motion (stationary, orbiting, slow). The erosion rates of various cathode materials (aluminium, gallium, molybdenum, and tantalum) were measured in both diffuse and constricted, magnetized He plasmas. The constricted orbiting (CO) mode exhibited a lower erosion rate than either the diffuse stationary or constricted slow modes, even when comparing loss rates against a cathode material that was more liable to sputter (e.g. aluminium versus molybdenum). However, the erosion rate for operation in the CO mode was still found to be significantly higher than required for practical use in a gas discharge switch.

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